THE POTENTIAL FOR HIGH RESOLUTION PALEOCLIMATE RECONSTRUCTIONS AT LAGUNA MINUCUA, OAXACA, MEXICO

We present preliminary data from Laguna Minucua, located within the Sierra Madre del Sur, Oaxaca, at an elevation of ~2500 m. Laguna Minucua is a small (~0.25 ha), currently shallow (<30 cm deep at time of coring) pond with no apparent inlets or outlets. The pond likely formed in a carbonate sink but this is not verified. The site is surrounded on the northwest side by a carbonate ridge, which has stands of Pinus oaxacana and Quercus spp. growing on the slope. We retrieved two sediment cores from the site (3.5 m and 5.6 m long). Surprisingly, given the current shallow water depth, the cores are highly laminated throughout the core length and are possibly varved.

The laminae were characterized using micro-morphological analysis from thin sections, high resolution (200 micron) scanning XRF and magnetic susceptibility data. The laminae show a clear fining upwards sequence. The dark laminae consist of coarser grained sand-sized crystalline material that grades upwards into the much finer and lighter colored clay-sized laminae. The contact points between the light and dark couplets are abrupt. Scanning XRF data indicate an inverse relationship between sulphur (peaks in the lighter laminae) and titanium (peaks in the dark laminae). The thin section analysis and XRF data suggest that the couplets may reflect annual deposition. We tentatively speculate that the fine grained light-colored layers represent phases of higher evaporation in winter and spring when rainfall is limited and the graded darker layers represent higher summer rainfall, with increasing organic/clastic input and gradual settling of material from suspension.

Core chronology was developed with paleomagnetic secular variation data and compared with varve measurements. These data indicate that the Minucua record spans the last ~4,000 years. A preliminary paleoclimate reconstruction for a 500 year time slice between ~550-1100 A.D. is discussed in more detail as this represents a critical time in Mesoamerican cultural development.